1. Field of the Invention
The present invention relates to a method for reducing the backlash sound in a gear mechanism, a power output apparatus having such a gear mechanism, and a hybrid vehicle equipped with a power output apparatus. More particularly, the invention relates to a technology for reducing the backlash sound in a power output apparatus that includes an internal combustion engine as a power source, a gear mechanism having at least three shafts, one of which is connected to the output shaft of the internal combustion engine and another one of which is connected to a drive shaft, and an electric motor connected to still another one of the at least three shafts.
2. Description of the Related Art
For highly efficient operation of an internal combustion engine and a remarkable improvement of emissions from the engine, hybrid vehicles have been proposed which are equipped with a power output apparatus that performs power transmission between an electric motor and a drive shaft and/or an internal combustion engine using a planetary gear unit, in addition to the conventional power transmission from the internal combustion engine to the drive shaft. In a typical power output apparatus, the output from the internal combustion engine is distributed to the drive shaft and the electric motor by the planetary gear unit so that energy transmitted to the electric motor is regenerated as electricity and, in another occasion, the power from the internal combustion engine and the power from the electric motor are combined and outputted to the drive shaft by the planetary gear unit. Furthermore, during braking, the electric motor is operated to regenerate electric power, thereby applying a braking force to the drive shaft. The electric power regenerated by the electric motor is stored into a battery or the like, and it is used to operate the electric motor when necessary. Therefore, when it is possible to drive the vehicle only by the electric motor, the internal combustion engine can be stopped. With regard to this type of power output apparatus, a technology has been proposed in which in addition to the electric motor as described above (first electric motor), another electric motor (second electric motor) is connected to the drive shaft side, and the driving and regeneration by the first and second electric motors is controlled so that the vehicle can be driven in various driving modes. For example, these modes may include an assist mode in which the drive shaft is rotated at a low speed and a high torque by using the first electric motor to regenerate electric power and using regenerated electric power to drive the second electric motor, an over-drive mode in which the drive shaft is rotated at a high speed and a low torque by using the second electric motor to regenerate electric power and driving the first electric motor, an accelerating mode in which a high accelerating performance is achieved by driving both electric motors, a braking mode in which at least one of the electric motors is used to regenerate electric power and a braking force corresponding to the regenerative energy is applied to the drive shaft, and the like.
An electrical type torque converter is also proposed in which the output shaft of an internal combustion engine is connected to a shaft of a planetary gear unit, and another shaft of the planetary gear unit is connected to an electric motor. In this type of torque converter, the output shaft of the internal combustion engine is connected to, for example, a ring gear shaft of the planetary gear unit, and a sun gear shaft of the planetary gear unit is connected to the electric motor, and a carrier shaft thereof is connected to the drive shaft or an input shaft of a transmission or speed shift device that is connected to the drive shaft. In this construction, when conduction of electric current through three-phase coils of the electric motor is prevented (no-load condition), the carrier shaft idles without outputting power during operation of the internal combustion engine. If the condition is switched by controlling the drive circuit of the electric motor so that current through the three-phase coils will gradually increase from zero and, therefore, regeneration of electric current is started, a braking force corresponding to the regenerated current occurs on the sun gear so that the drive shaft receives a torque that is, at maximum, (1+.rho.)/.rho. times the output torque of the internal combustion engine where .rho. is the gear ratio of the planetary gear unit.
In a power output apparatus or a torque converter as described above, gear teeth backlash sounds, also called clattering sounds, may be produced in the gear mechanism such as the planetary gear unit and the like. Such backlash sounds occur because there is a small clearance in the mesh between teeth of gears and, therefore, teeth of one of the gears in mesh repeatedly strike and separate from teeth of the other gear at the time of a change of the gear driving force and the like. The gear clearance is designed to be as small as possible. Since elimination of the gear clearance results in tight interlocking between the gears thereby making it impossible for the gears to rotate, a small gear clearance is essential in normal gears. As a measure for preventing the strike and separation between gear teeth, a scissors gear construction is known in which teeth of meshing gears are clamped. In this construction, however, the clamping of teeth becomes a braking force against rotation of the gears, giving rise to a problem of energy loss. Considering that the power output apparatuses as described above are intended to improve the energy efficiency of the entire apparatus, it is difficult or inappropriate to employ a gear mechanism having a design that produces a predetermined energy loss.